1. Field of the Invention
The present invention relates to a sewing machine having a function of tracing the edge of a workpiece, and more particularly to such an edge tracing sewing machine equipped with an edge sensor which is adapted to be movable in a direction perpendicular to a feed direction of the workpiece, for detecting the workpiece edge.
2. Discussion of the Prior Art
An edge tracing sewing machine is known according to laid-open Publication No. 61-257675, the subject matter of which is incorporated in U.S. Pat. No. 4,664,048. In this sewing machine, the edge of a workpiece or work fabric is detected by an arrangement which includes (i) an edge sensor which is movable in a direction (lateral direction) perpendicular to the feed direction of the work fabric, (ii) a control motor for moving the edge sensor in the lateral direction, and thereby changing a relative position between the sensor and the fabric, and (iii) detector position control means for controlling an operation of the control motor so as to adjust the lateral position of the edge sensor so that a value of an output signal of the edge sensor coincides with a predetermined reference value, namely, so that the sensor is aligned with the edge of the fabric. Based on the detected edge position of the fabric, the lateral position of a sewing needle is regulated by a position control device. That is, a distance in the lateral direction between the fabric edge and a line of stitches to be formed is determined by a lateral distance between the fabric edge position and the needle position.
In the edge tracing sewing machine of the type described above, the reference position for positioning of the sewing needle (hereinafter simply referred to as "needle positioning") is determined by the position of the edge sensor in the lateral direction. To position the edge sensor in alignment with the fabric edge, a position control signal representative of the lateral position of the sensor is applied to the control motor.
The control motor, and a connection or linkage between the control motor and the edge sensor have mechanical gaps or clearances provided for smooth sliding or rotating actions. These gaps or clearances result in an operating play, which causes a deviation or error between the commanded and actual positions of the edge sensor. If the direction of such a deviation were fixed irrespective of the operating direction of the moving members involved, the deviation can be easily compensated for by shifting the edge sensor in the direction in which the deviation can be eliminated. In fact, the operating direction is reversed, and the direction of the deviation or error is also reversed. Accordingly, the actual position of the edge sensor corresponding to a given value of the position control signal applied to the control motor differs with the direction of scanning the fabric edge position, even if the edge position of the fabric detected by the sensor is the same. Consequently, the known arrangement suffers from a lateral positioning error of the sewing needle relative to the fabric edge, which results in inaccurate tracing of the fabric edge by the sewing needle.